CTM640 - Unstable voltage

[jelcynek]

Hello,
Lately I try to repair my CTM640 and need advice from some more experienced users. So, the case is that in general it works flawlessly. Colors are vivid, picture is crisp. The only problem is that it gets random voltage drops which are visible as short snaps on screen and drops also occurs on 5V what cause computer to hang up and show garbage on the screen. It happens once very few or dozen seconds. 

Here are videos from oscilloscope showing short voltage drops on 5V (around 40 second):
https://share.icloud.com/photos/038JU4iCuNlgOC0j0iGoToUCQ

and the display itself:
https://share.icloud.com/photos/0ceHkV7FwlW8QJTzo5ft_p4GQ

I opened display, discharged it and went through all of the boards and electronics. Judging by the look everything looks fine. No puffed or leaked capacitors, no burn marks. So right now, my further actions would be to measure voltages on the board with service manual but I'm a little afraid of that or to desolder element by element and check with tester if everything is right. 
v
But first I decided to write here. For my limited electronic knowledge, it looks like maybe some capacitor responsible for filtering voltage is broken. Any ideas?
 

[pelrun]

Just because the electrolytic capacitors aren't visibly damaged doesn't mean they're in good shape. It's almost certain that most of them are dead or on their last legs, simply from age.

[jelcynek]

Yes. I'm aware of that. But before I will start replacing every capacitor, I thought that maybe there are the ones which may be more responsible for that behavior than other. Thus, why I asked here. You know, like a hint where on the schematics looks closer and what to check before I start replacing everything.

[Bryce]

Hi,
    is there also an audible "snap" when the voltage drops?

Bryce.

[jelcynek]

is there also an audible "snap" when the voltage drops?

Yes. Right now, I use power supply for 5V for computer and everytime when there is a voltage drop on the display there is an audible "pop" or "snap" and on the display it can be seen as well. Video also makes snap. 

[darkhalf]

Just because the electrolytic capacitors aren't visibly damaged doesn't mean they're in good shape. It's almost certain that most of them are dead or on their last legs, simply from age

Sort of agree with this one. It depends on the capacitors. I've certainly had some devices with bad capacitors (including 90's motherboards, STB boxes, and Toyota ECUs) which have expanded or leaked and need replacing

However there are also other brands which are still showing good ESRs almost 40 years later. So it depends on the brand and quality of the capacitor. Using an ESR meter to check is the best way to know for sure. 

There is nothing worse than sourcing and then replacing parts, later knowing that it was for nothing

What I do know with going through the pain of repairing two Amstrad monitors is I've checked all the caps ESRs and they were fine. Also with this type of problem, we are seeing an intermittent spike/drop in voltage associated with crackling. That doesn't really appear capacitor related IMHO

everytime when there is a voltage drop on the display there is an audible "pop" or "snap"

ATM I'm calling the flyback on this one or there is a dry joint which is causing the spark (check and reflow joints)

If you search for CTM640 or look through the threads in this section, you will see I've made a few posts on both CTM640 and CTM644 monitors which died on me. 

So there is a STK voltage regulator which provides regulated power to the rest of the monitor. There are two power rails which come out of this. What is more interesting is there is power from the flyback which goes into the 5V regulator. So since you are seeing the intermittent voltage drop from the regulator, but also hearing a crack, it leads to that crack coming from something that is starting to fail (transformer, regulator, flyback)

With flybacks there is a place Tulli Electronics which has these for a decent price. These are the last (in the world!) that I know exist and there appears to be 3 remaining
https://www.tullielettronica.net/prodotti/hr6355-145961.aspx
So my money is on flyback, given I've had to replace 3 in monitors in the last year. The coils vibrate, enamel flakes and they short out

[MoteroV4]

I recently repaired an intermittent fault in a CRT where old solders made bad contact. As they reach temperature, certain components (especially power components) tend to expand microfractures not visibles.

[jelcynek]

Slow update

I bought flyback from Tulli Electronics. Before changing it I resoldered all pins on flyback but it didn't help. So now the patient lays on my operation table ready for the flyback transplantation. 

I didn't find any video of changing it so first I am coming to you for an advice. I desoldered all of the flyback pins and the big ground one at the middle. Unscrew three screws. Is there anything more I need to do before I will start pulling flyback? 

[Bryce]

is there also an audible "snap" when the voltage drops?

Yes. Right now, I use power supply for 5V for computer and everytime when there is a voltage drop on the display there is an audible "pop" or "snap" and on the display it can be seen as well. Video also makes snap.

Then it could be cracks in the cable from the flyback to the CRT. You could try putting an additional insulator around the cable to see if that resolves the issue. Cut a slit along the length of some silicon tubing and put it over the full length of the red cable. Then use electrical tape to close the tubing from top to bottom.

Bryce.

[jelcynek]

With flybacks there is a place Tulli Electronics which has these for a decent price. These are the last (in the world!) that I know exist and there appears to be 3 remaining
https://www.tullielettronica.net/prodotti/hr6355-145961.aspx
So my money is on flyback, given I've had to replace 3 in monitors in the last year. The coils vibrate, enamel flakes and they short out

darkhalf you replaced flybacks for the ones from the Tulli Electronics? I wonder, because the one from the Tulli Electronics doesn't have that thick wire which was connected originally to the Ground. How did you managed that? Was it only to keep flyback in place?

[jelcynek]

Nevermind. I just realized there is other pin connected to the GND in the flyback. So I suppose that big wire was only to keep the flyback in place.

[Bryce]

Nevermind. I just realized there is other pin connected to the GND in the flyback. So I suppose that big wire was only to keep the flyback in place.

Or to give it a direct path to the metal frame (and earth) in case there's a short and extremely high current burns the tracks off the PCB? Is there a picture of the mystery "big wire" somewhere?

Bryce.

[darkhalf]

darkhalf you replaced flybacks for the ones from the Tulli Electronics? I wonder, because the one from the Tulli Electronics doesn't have that thick wire which was connected originally to the Ground

The part HR6355 (from Tulli) has the red annode plug and a black (ground wire) also. I've ended up with a spare and just checked it now. The only thing I recall different was no screw mounts to the PCB

[jelcynek]

Wire is incorrect word for that. I mean the metal rod/stick over metal core. It was soldered to the big soldered pad to the GND and this doesn't exist on the HR6355.

Like on this picture: https://1.bp.blogspot.com/_YbD_csZtad8/Swf2Z_dIBdI/AAAAAAAAAeE/Tdfddmns-6c/w1200-h630-p-k-no-nu/flyback_pic_01.jpg

[jelcynek]

[deleted]

[jelcynek]

Replacing flyback doesn't helped. Still it gets voltage drops.

[Bryce]

Replacing flyback doesn't helped. Still it gets voltage drops.

Try running the monitor (without the CPC connected) with the case open in a dark room. When the voltage drops you'll either see a flash at the back of the tube, which would mean there's arching on the board, or... if the flash can be seen in the CRT from the front, then the tube itself is failing.

Bryce.